You don’t pay tax on currency movement while you are invested so you must do all the calculations in USD such as the purchase and sale plus the profit/loss. Only then do you convert the USD to ZAR at the spot price on the day the trade is concluded.This amount must be recorded by you for efiling later. Note that all amounts in your broker account must remain in USD for all future trades. Only once the trade is concluded do you record the profit/loss in ZAR.